1. Field of the Invention
This invention relates to an apparatus for controlling the output of a Stirling engine.
2. Description of the Prior Art
The output of a Stirling engine, which is an engine of the external combustion type, is determined by the pressure in a working space in which a working gas is sealed. For example, when it is desired to raise the output of a Stirling engine, the pressure of the working gas in the working space is raised. A typical prior-art output control apparatus for a Stirling engine, such as described in the specification of Japanese Patent Application Laid-Open No. 46-23534, is shown in FIG. 1. Each working space 1 of the engine is connected via a check valve 2 to a compressor 3 by way of a maximum cycle pressure line 4. The line 4 has a pressure reducing valve 5. Each working space 1 is also connected via another check valve 6 to the the compressor 3 by way of a minimum cycle pressure line 7, which has a pressure boost valve 8. Numeral 9 denotes a high pressure tank.
The downstream side of the pressure boost valve 8 is connected to a feedback piston cylinder 10 having a piston 10a connected via a piston rod 10b to one end of an accelerator lever 11, which serves as an operating lever. A valve stem 12 of the pressure boost valve 8 and a valve stem 13 of the pressure reducing valve 5 are disposed in facing relation with respect to the sides of the accelerator lever 11. The piston 10a is moved as a function of the pressure in the feedback piston cylinder 10 in such a manner as to shift the position of a fulcrum 14 of the accelerator lever 11.
When it is desired to raise the output of the Stirling engine, the accelerator lever 11 is pushed leftwards to open the pressure boost valve 8 in order to supply the pressurized working gas to the working space 1 from the compressor 3 or tank 9. Conversely, when it is desired to lower the engine output, the accelerator lever 11 is pushed rightwards to open the pressure reducing valve 5 in order to vent the pressure in the working space 1 toward the compressor 3 and lower the pressure within the working space 1.
The amount of movement of the rod 10b of feedback piston cylinder 10 is proportional to the pressure in the minimum pressure cycle line 7. Therefore, in a case where a grip portion 11a on the accelerator lever 11 is held at a constant position for acceration, the position of the fulcrum 14 of the accelerator lever 11 is shifted gradually rightward according to the pressure in the minimum pressure cycle line 7. As a result, although the accelerator lever 11 is held at said constant position for acceleration, the pressure boost valve is likely to close its opening. During the time that the boost valve makes the transition from the slightly open state to the fully closed state, passage of the working gas is allowed to slowly continue. The valve opening fluctuates for several tens of seconds before settling down.
As a result of this phenomenon, engine torque gradually rises even if the accelerator lever is at a constant position. If the load is constant, therefore, engine rpm will rise. The present invention is directed to solving this problem.